In backlight computer displays or other display systems, optical film (which may also be referred to as a sheet, layer, foil and the like) materials are commonly used, for example, to direct, diffuse, or polarize light. For example, in backlight displays, brightness enhancement films (BEFs) use prismatic structures on the surfaces thereof to direct light along a viewing axis (i.e., an axis normal (perpendicular) to the display). This enhances the brightness of the light viewed by the user of the display and allows the system to consume less power in creating a desired level of on-axis illumination. Such films may also be used in a wide range of other optical designs, such as in projection displays, traffic signals, and illuminated signs.
In current displays systems, for example in liquid crystal displays (LCD), it is desirable to have light-diffusing films. Light-diffusing films describe a broad class of articles that are used within LCD backlight systems to evenly distribute light to the viewer and hide potential defects generated by a light guide, while maintaining total transmission of light. In addition to these light management properties, these diffusing films need to satisfy certain properties related to their visual appearance in a backlight display system. In particular, these films should lay flat on other films in the system. Generally, when polycarbonate films are used in this application, their high coefficient of thermal expansion may cause portions of the film near the hot lamp to expand while film further from the lamp either does not expand or does not expand to a similar extent, resulting in waviness of these portions of the film, making the entire film unsuitable for use in a backlight display system.
What is needed in the art is a light-diffusing polycarbonate film that minimizes or does not result in observable waviness under the heat of a lamp in backlight system.